1. Field of the Invention
This invention relates to improvements in an internal combustion engine of the split type operable on less than all of its cylinders when the engine load is below a given value.
2. Description of the Prior Art
As is known it is desirable to increase the efficiency of a multicylinder internal combustion engine by reducing the number of cylinders on which the engine operates under predetermined engine operating conditions, particularly conditions of low engine load. For this purpose, control means are provided which disables a number of cylinders in a multicylinder internal combustion engine by suppressing the supply of fuel to certain cylinders or by preventing the operation of the intake and exhaust valves of selected cylinders under low load conditions. The disablement of some of the cylinders of the engine increases the load on those remaining in operation and, as a result, the energy conversion efficiency is increased.
One difficulty with such split type internal combustion engine is the tendency toward unstable engine operation and increased engine vibration during a split engine mode where the engine operates only on selected cylinders and the interval between which explosion occur becomes larger. This is true particularly at low engine speed conditions.
In order to achieve stable engine operation and suppress engine vibration, attempts have been made to maintain engine operation in a full engine mode (where the engine operates all of the cylinders), regardless of engine load conditions, whenever the engine speed is below a predetermined value. However, such operation results in a fuel economy penalty since ,he engine is held in a full engine mode of operation at low engine speed conditions, regardless of the actual occurrence of unstable engine operation.
The present invention provides an improved split type internal combustion engine wherein engine operation to changes into a full engine mode, regardless of engine load conditions, whenever the engine vibration level exceeds a value indicating unstable engine operation.